Signal transmission system



` M .EVEWL jig? E. G. EJORNSUN SIGNAL TRANSMISSION SYSTEM Filed Jan. 26,1940 2 Sheets-Sme?J AAA www

A T TUM/VE' V im? M'LI.. BJQRNmN SIGNAL TRANSMISSION SYSTEM Filed Jan.26, 1940 2 Sheets-Sheet 2 Arrow/EV Patented Mar. 18, 1941 UNITED STATESPATENT OFFICE SIGNAL TRANSMISSION SYSTEM Application January 26, 1940,Serial No. 315,679

13 Claims.

This invention relates to signal transmission systems and particularlyto signal transmission systems having switching circuits governed by thesignal energy.

One object of the invention is to provideasignal transmission line withswitching circuits that shall operate Without signal clip-ping tocontrol the line when signals are impressed on the line.

Another object of the invention is to provide a signal transmission linewith control circuits to maintain constant volume on the line andswitching circuits that shall operate without signal clipping toclearthe line when signals are impressed on the line.

A further object of the invention is to provide a signal transmissionline having a gain control device therein with control circuits forgoverning the device to maintain constant volume on the line and withswitching circuits operated by the 20 signals on the line before saiddevice without signal clipping to clear the line when signals areimpressed on the line.

In asignal transmission system provided with singing suppressor circuitsthe transmission path or line is provided with switching circuits forblocking the transmission line when no signals are impressed on the lineand for removing the blocking means when signals are impressed on thetransmission line. Under certain conditions a sudden change in thestrength of the signals transmitted over the transmission line may causethe so-called clipping. In a signal system having a gain adjustingdevice on a transmission line operated to hold the signal volumeconstant, it is desirable to have a comparatively fast gain decrease incase of high amplitude signals to lessen the probability of overloadingapparatus on the transmission line. If high amplitude signals arefollowed by low amplitude signals and the high amplitude signals arequickly reduced by the gain adjusting device then there is danger ofsignal clipping by the switching circuits which control the blocking oithe transmission line.

According to the present invention, the signal currents which operatethe switching circuits to clear a normally blocked transmission line arecontrolled to insure against any signal clipping. The signal currentswhich operate the switching circuits are adjusted to energy levels whichinsure against clipping.

In `one form of the invention a gain varying device on a signaltransmission line is controlled to maintain substantial constant volumein the manner disclosed in the patent to B. G. Bjornson 2,173,816,September 26, 1939. According to the Bjornson Patent 2,173,816, it hasbeen discovered that when a speaker talks with constant volume twodifferent energy levels are exceeded different per cents of time andthat the ratio of the rdifferent per cents is approximately constant.Moreover, the volume of a speaker may be measured within certain limitsby determining the durations oi two different energy levels in speech.Two different energy levels El and E2 in speech may be taken which areexceeded respectively 10 `t1 and t2 per cent of the time. A ratio ti/tzwhich is approximately constant may be found which will give the desiredsignal volume.

In practicing the invention disclosed in the Bjornson Patent 2,173,816,the gain control de- 15 'vice in the `transmission line is governed sothat the two energy levels in speech E1 and E2 are exceeded only t1 andtz per cent of the time respectively. A balancing condenser is chargedby potentials oi opposite polarity under control of two detectorcircuits which are connected to the transmission line beyond the gaincontrol device therein in any suitable manner, as by means of a hybridcoil. Each detector circuit comprises a marginal space discharge devicewhich is preferably in the form of a three-element gas-filled tube. Oneof the marginal devices is set to operate or trigger oil when the energylevel E1 is on the transmission line and the other marginal device isset to operate when an energy level E2 is on the transmission line. Tworelays are respectively operated by the two marginal devices. .One ofthe relays, which is operated by the detector device when a level of Elobtains on the transmission line, completes a circuit for charging thebalancing condenser by potential of one polarity from a battery througha resistance for example, of 20,000 ohms. The other relay which isoperated by the other detector device when a level of E2 obtains on thetransmission line completes a circuit for charging the balancingcondenser by potential of opposite polarity from a battery through aresistance for example, of 160,000 ohms.

The resistance elements which are included in the charging circuits forthe balancing condenser are of such size that the condenser is chargedat rates inversely proportional to the times the voltages E11 and E2 areexceeded. In the arrangement under consideration let 0 Ts=the operatedtime of the relay operated by voltage V2 and lTA==the operated time ofthe relay operated by voltage V1. :i5

Assume 20,000 ohm resistance is r and 150,000 ohm resistance is R.

rllhen the relation between r and R is and Thus, if the detector deviceoperated by the voltage V1 is operated 8 per cent of the time, thedetector device operated by the voltage V2 may be operated '12 per centof the time.

When the charge on the balancing condenser is raised above apredetermined value by a charge of either polarity a transfer relay isoperated to change the charge on a control condenser. The controlcondenser governs the gain control device in the transmission line. Ifthe balancing condenser is charged with one polarity under control ofthe relay operated by the high voltage V1 then the transfer relay willbe operated to impress a negative charge on the control condenser toincrease the loss` effected by the gain control device. If the balancingcondenser is charged above the predetermined value with oppositepolarity under control of the relay operated by the lower Voltage V2then the transfer relay will be operated to discharge a portion of thenegative charge on the control condenser to decrease the loss effectedby the gain control device.

The switching circuits employed to describe the invention comprise anamplifier detector connected to the transmission line before the gainvarying device therein. The amplier detector controls a relay whichserves to block the transmission line beyond the gain varying devicewhen no signals are on the transmission line. The signal currents whichoperate the amplifier detector are governed by auxiliary circuitssimilar to those shown in the Bjornson Patent No. 2,173,816 wherein theresistance elements r and R are changed to insure against any clippingof the signals on the transmission line by the relay operated by theamplifier detector. In one arrangement the resistance r is 60,000 ohmsand the resistance R. is 160,000 ohms. In this case the relation betweenTA and TB is In the arrangement under consideration for controlling thesignal currents on the switching circuits the detector device operatedby the voltage V1 may be operated 20 per cent of the time and then thedetector device operated by the voltage V2 may be operated 13.2 per centof the time. The lower amplitude signal currents will be raised so thatcontrol of the blocking relay can be effected at all times when signalsare on the transmission line. In the example under consideration, thevalue of the resistance 1' was increased. A similar result could beeffected by reducing the value of the resistance element R.

In the accompanying drawings,

Figs. 1 and 2 taken together with Fig. 1 above Fig. 2 disclosetransmission circuits constructed in accordance with the invention; and

Fig. 3 is a diagrammatic View of a modification of the circuits shown inFigs. 1 and 2.

Referring to Fig. 1 of the drawings, a transmission line is connected toan attenuation control station by input conductors l and 2 and outputconductors 3 and 4. A vario-repeater 5 in the form of tvvo spacedischarge devices 6 and 'i is connected to the input conductors l and 2by a transformer 8 and to the output conductors 3 and 4 by transformer9. The device 6 is shown in the form of a pentode and is provided with acathode i0, a control grid H and an anode l2. The device 1 is in theform of a pentode havingr a cathode I3, a control grid I4 and an anodel5. The vario-repeater 5 is effectively connected in series with thetransmission line and is governed to control the gain so as to maintainsubstantially constant volume on the output conductors and fl. A fixedminimum. biasing potential for the grids H and i4 of the devices 6 and 1is supplied by a battery i6. A variable biasing potential for thecontrol grids II and I4 of the devices t and 1 is` supplied by a controlcondenser i1. The charge on the control condenser S1 is governed in amanner to be hereinafter set forth for controlling the vario-repeater 5to govern the volume of the signals on the transmission line. A batteryI8 is connected to the anodes l2 and l5 of the devices 6 and 1 throughresistance elements IS and 20.

rEvvo detector circuits 2i and 22 are connected to the output conductors3 and 4 of the transmission line by a hybrid coil 23. The detectorcircuit 22 comprises a three-element gas-lled tube 22. The input circuitof the tube 24 is connected to the hybrid coil 23 by means of atransformer 25. The output circuit of the tube 5 is connected in serieswith a coil 25 of a polarized relay 21. A condenser 28 associated with aresistance element 29 and an inductance element 30 is provided to insurea hang-over in the operation of the relay 21 by the tube 24. Thehang-over effected by the condenser resistance arrangement is preferablyof the order of .02 second. A battery 3l is provided for supplying anodepotential to the anode of the tube 24. A battery 32 is provided forsupplying filament heating current and grid bias to the tube 2li.

The relay 21 is provided with a biasing winding 33 which is connectedthrough a resistance element 34 to the battery 3|. An armature 35 of therelay 21 which is operated by the coil 26 engages contact members 35vand 31. In the released position of the relay 21 when the coil 25 is notenergized the armature 35 engages the contact member 31.

The detector circuit 22 comprises a threeelement gas-filled tube 38which is marginal in operation and an amplifier 39 of any suitable type.The input circuit of the amplifier tube 39 is connected by a transformer40 to the hybrid coil 23. The output circuit of the amplifier tube 39 isconnected by transformer 4| to the input circuit of the detector tube38. The output circuit of the detector tube 38 is connected in serieswith the operating winding 42 of a polarized relay 43. A condenser 44associated with a resistance element 45 and an inductance element 46 isprovided for insuring a hang-over in the operation of the relay 43 ofthe detector tube 32. The hang-over time in the operation of the relay43 is of the order of .02 second. A battery 41 is provided for supplyinganode potential to the detector tube 38 and for energizing a biasingwinding 43 on the relay 43. The biasing winding `48 is connected to thebattery 41 in series with a resistance element 49. A battery 50 suppliesanode potential to the amplifier tube 39. A battery 5I supplies lamentheating cul*- rent and grid biasing potential to the detector tube 38and the ampliner 4tube 39. The relay llt is provided with an armature 52which is adapted to engage contact members 53 and 54. When the operatingcoil 42 of the relay 43 is deenergized the; armature 52 is held engagedwith the contact member 53.

The relay 43 in combination With the relay 2'! controls the chargeimpressed on a. balancing condenser 55. 'Charges of opposite polarityare impressed on the balancing condenser 55 from batteries 56 and 51through resistance elements 5t and 59. The batteries 56 and 51arepreferably of the same voltage and are of the order of one andone-half volts. The resistance elements 50 and 59 may be of the order of20,000 ohms and 160,000 ohms, respectively. With the relay 2l inreleased position and the relay 43 in operated. position the balancingcondenser 55 will be charged with potential from the battery 5l incircuit with the relatively large resistance 50. When the relay 21 is inoperated position the balancing condenser 55 is charged from the battery50 in circuit with the relatively small resistance 58.

The detector circuits 2l and 22 are operated by different energy levelsof speech currents on the transmission line. The detector circuit 22 isassumed to be operated at a lower level, for example, 12% decibels andthe detector circuit 2| is assumed to be operated at a higher level, forexample, +10 decibels. The difference in sensitivity between the twodetector circuits is 22.5 decibels.

A transfer relay 00, which is controlled by the balancing condenser 55,serves to control the charge on the control condenser Il. The transferrelay 50 comprises an operating Winding 6|, a biasing winding 62 and anarmature 53 which is adapted to engage contact members 64 and 55. Theoperating Winding I of the relay E0 is connected to the output circuitof an amplifier tube 50. The amplier tube 66 is preferably in the formof a three-element space discharge device. Biasing potential for thegrid of the amplifier tube 50, is supplied by a battery 61 and thebalancing condenser 55. A battery B8 is provided for supplying anodepotential to the amplifier tube 00 and for energizing the neutralizingWinding 02 of the relay 60. The circuit for the winding 02 extends fromone terminal of the battery 50 through the winding 62 and resistanceelements 50 and 'l0 in parallel to the other terminal of the battery 50.The resistance element 69 has a non-linear coefficient of resistance andpreferably is composed of silicon carbide crystals and a binder materialor, copper-oxide rectier elements. Resistance elements of this type aredisclosed in the patent to K. B. McEachron 1,822,742, September 8, 1931.

Normally with no charge of either polarity on the balancing condenser 55the eiects of the operated Winding l and the neutralizing winding 0i ci'the relay 00 are neutralized and the armature 55 is held midway betweenthe contact members 'it and 65. When the relay 2l is operated by thedetector circuit 2l the balancing condenser 55 is charged with potentialof negative polarity from the battery 55. When the negative charge onthe balancing condenser 55 is raised to a pre determined point thenegative bias on the grid ofthe ampliiier tube 56 reduces the platecurrent flow through the winding 0| of the relay 60 to move the armature63 into engagement with the contact member 55. When the relay 43 isoperated by the detector circuit 22 and the relay 2l is in releasedposition, the balancing condenser 55 is charged with potential ofpositive polarity from the battery 5l. Upon charging of the condenser 55to a predetermined point by the battery 5l, the positive potential biason the grid of the amplier 65 increases the plate current iiow throughthe winding iii of the relay 60 to move the armature 55 into engagementwith the contact member 1f the charge on the balancing condenser 55 isinitially Zero, the armature 35 of the relay 2l will have to be left onthe contact member 35 a deiinite length of time, for example .06 second,to allow the transfer relay t0 to move the armature E3 into engagementwith the Contact member 55. When the armature 53 engages the contactmember 55 the gain on the transmission line is reduced, as will beexplained hereinafter.

If the charge on the balancing condenser 55 is aero and the armature 52of the relay l5 is moved into engagement with the contact member 54, itwill take a denite length of time, for example, .15 second, before thearmature 53 of the transfer relay 50 may be moved into engagement withthe contact member (ifi. When the armature 63 of the transfer relay 50is moved into engagement with the contact member 55, the gain on thetransmission line is raised, as will be explained hereinafter.

A relay 'il comprising an operating coil 'l2 and an armature 'i3 isprovided for insuring against change in the charge Von the controlcondenser il except when speech currents are on the transmission line.The coil l2 of the relay 'll is normally short-circuited by the armature52 of the relay 43 and the armature i3 is held in engagement with a backcontact member l5. Upon operation of the relay 43 the coil l2 of relay'H is energized from a battery lli and the armature 13 is moved intoengagement with the contact member ll for placing a ground on thearmature 53 of the transfer relay till. Normally a shunt circuit ismaintained around the battery 'i4 and a resistance element 'l5 to insurethe release of the relay l'l. The shunt circuit around the battery 14and the resistance element 'i5 may be traced from one terminal of theresistance element 'l5 through contact 53, armature 52 and ground returnto a terminal of the battery M. Upon operation of the relay 43, theshunt connection around the battery 'M is opened so that the relay ll isoperated whenever the relay 43 is opl erated. The relay i3 is operatedby the detector circuit 22 of higher sensitivity.

When the armature 63 of the transfer relay 50 is moved into engagementWith contact member 6d the control condenser il is discharged through aresistance element 18. The discharge circuit for the control condensermay be traced from one terminal of the condenser Il through theresistance element T8, contact member 64, armature 53, contact memberll, armature l5 and ground return to the other terminal of the condenserIl. When the armature t3 is moved into engagement with the contactmember 65 the control condenser Il is charged with potential of negativepolarity from a battery l0. The charging circuit for the controlcondenser il may be traced from one terminal of the condenser il througha resistance element 80, battery 79, contact member 55, armature 63,contact member l1, armature 13 and ground return to the other terminalof the condenser il. The sizes of the resistance elements 'i8 and 56 maybe varied as desired. The rate of gain increase depends upon the valueof the condenser i l and the resistance I8. The rate of gain decreasedepends upon the condenser IT, resistance element 65 and the battery l5.

Assume that a weak talker transmits speech current over the transmissionline with a low volume. Under this circumstance, the relay 43 will beoperated by the detector circuit 22 and the relay 2l will not beoperated. After two or more syllables, depending on the previouscondition of the system and the input amplitude, the relay 55 will movethe armature 63 into engagement with the contact member 64. When therelay i5 is operated the balancing condenser 55 is charged withpotential of positive polarity from the battery l. The lcircuit forcharging the balancing condenser 55 may be traced from one terminal ofthe condenser through the armature 35 of the relay 2l', contact member3T, resistance element 53, battery 57, contact member 54 and armature 52of the relay 43 to the other terminal of the balancing condenser 55. Thecharge on the balancing condenser 55 impresses positive potential on thegrid of the amplifier device 56 to increase the current flow through thedevice. This eiiects operation of the transfer relay 60 to move thearmature 63 into engagement with the contact member 54. The negativecharge on the control condenser Il is reduced by a circuit including theresistance element 7B. The relay 'H is operated at this time inasmuch asthis relay is operated whenever the relay 43 is operated. A reduction inthe negative bias on the control grids l i and lf3 of the tubes 5 and 1increases the gain cn the transmission line. It may be noted that therelay l, which is operated only when the relay 43 is operated insuresthat change in the charge on the control condenser l1 can only takeplace when speech currents are on the transmission line.

The circuits controlled by the relays 21 and 43 are timed so that whenoperated the balancing condenser 55 is charged with potentials ofopposite polarity and approximately inversely proportional to thedurations of the operations of the relays. The gain on the transmissionline will be increased gradually until the relay 21 is operated. Therelay 2l is operated when the energy level on the detector circuit 2l israised above the high predetermined level. Upon operation of the relay27 the charging circuit of the balancing condenser 55 from the battery57 is broken and a circuit is completed from the battery 55 for reducingthe positive charge on the condenser 55. When the condenser 55 ischarged with potential of negative polarity to the critical point,negative bias is impressed on the grid of the device 66 to reduce thecurrent now through the winding 5I of the relay 66 sufiiciently to movethe armature 63 into engagement with the contact member 65. At this timethe relay 'H is in operation because the relay 43 is in operation. Therelay 43 is always in operation when the relay 21 is operated. Thecontrol condenser H is charged with potential of negative polarity fromthe battery 79 through the resistance element 80. Increase in thenegative charge on the control condenser Il increases in negative biason the grids I l and I 4 of the tubes 6 and 'I to lower the gain on thetransmission line. If the talker is speaking at substantially constantvolume it is possible to select the time constants of the circuit sothat the critical charge on the balancing condenser 55 is never exceededIf a critical charge of positive and negative polarity is never exceededon the balancing condenser 55, the relay 66 will not be operated and nochange will take place in the gain on the transmission line.

Switching circuits 8l, shown in Figs. 1 and 2, are connected between theinput conductors l and 2 and the output conductors 3 and 4 for blockingthe output conductors 3 and 4 whenever speech currents are off thetransmission line and for removing the line blocking when speechcurrents are impressed on the transmission line The switching circuits8l comprise a lter 82 which selects suitable frequencies within thevoice frequency range and a transformer 83 which connects the filter 82to a vario-repeater 84 similar in construction to the vario-repeater 5connected to the transmission line. The vario-repeater 84 comprises twospace discharge devices 85 and 86 which are preferably in the form ofpentodes. The devices 85 and 8G are controlled in a manner to behereinafter set forth for governing the level of the speech currentsemployed to operate the switching circuits. The output circuits of thedevices 05 and 86 are connected by a transformer el to anamplier-detector 83. The ampliierdetector 88 is connected to a relay 86which controls the blocking of the output conductors 3 and 4. The relay8S is provided with an armature 9G which in released position places ashort circuit across the conductors 3 and 4. The relay 89 is operated bycurrents supplied from the amplifierdetector 88 for operating thearmature 90 to remove the blocking of the transmission line when speechcurrents are being transmitted.

At times it may happen that the speech currents on the transmission linemay fall below amplitudes sucient to eiect operation of the relay 86which controls the blocking of the transmission line. It has beencustomary heretofore to connect the switching circuits to thetransmission line beyond the apparatus which maintains constant volumeon the transmission line. The apparatus which maintains constant volumeon the transmission line reduces high amplitudes rat-her quickly `andraises low amplitudes slowly. Thus, if high amplitude signals arefollowed by low amplitude signals it is apparent that the switchingcircuits under such conditions may block the line while signals arestill being impressed thereon. This will cause the so-called clipping ofportions of the signals. Such Conditions may also take place even if novolume control apparatus is employed.

In order to prevent any clipping of the signals by sudden changes inamplitudes of the signals being transmitted, the vario-repeater 84 iscontrolled by apparatus similar to the apparatus controlling thevario-repeater 5, to insure that the relay 8i! will always be operatedwhen signals are being impressed on the transmission line, The circuitsoperating the vario-repeater 64 are similar in construction andoperation to the circuits operating the vario-repeater 5 and accordinglylike parts have been indicated by similar reference characters with theaddition of prime marks.

In the circuits shown in Fig. 1 of the drawings the resistance elements58 and 59 have resistance values of 20,000 and 160,000 ohms,respectively. In the circuit shown in Fig. 2 of the drawings theresistance elements 58 and 59, shown in Fig. l, have been replaced byresistance elements 9| and 92 which have resistance values of 60,000ohms 'and 160,000 ohms, respectively. By changing the resistance Valuesof the resistance elements in the circuits controlled by the relay 27'it is possible to change the energy values of the control currentsbeyond the vario-repeater 84 which were supplied to the amplier-detectorB8 for operating the relay 89. A resistance value of 60,000 ohms for theresistance 80 is selected in place of the 20,000 ohms in order to effectelevation of the lower signal energies which are selected for operatingthe switching circuits 8|. The circuits shown in Fig. 2 for operatingthe vario-repeater 84 are operated in exactly the same manner as thecircuits shown in Fig. 1 for operating the vario-repeater 5, A detaileddescription of these circuits is deemed unnecessary. The detectorcircuits 2| and 22 shown in Fig. 2 of the drawings may be operated bythe same energy levels of speech current as the detector circuits 2l and22 shown in Fig. 1 of the drawings. The detector circuit 22' in Fig. 2may be assumed to be operated at a lower level, for example, 12%decibels, and the detector circuit 2| may be assumed to be operated at ahigher level,` for example, decibels.

Referring to Fig. 3 of the drawings, a modification of the circuitsshown in Figs. 1 and 2 is illustrated wherein two volume controlarrangements are Vemployed in combination with one set of switchingcircuits. A transmission line comprising conductors lili! and lill isillustrated having two volume control arrangements itt and 53 thereon.The volume control arrangements H32 and 93 may be similar to the volumecontrol arrangements shown in Fig. lof thedrawings. The volume controlarrangement I02 may effect constant volume rather roughly whereas thevolume control arrangement 03 effects substantially constant volume onthe transmission line. The switching circuits which control the blockingof the transmission line when no signals are impressed thereuponcomprise a filter 5M, which is similar to the filter 82 shown in Fig. loi' the drawings and which is connected to the transmission line beyondthe volume control apparatus |02. The filter lili is connected to gainadjusting apparatus 05 similar to the apparatus shown in Fig. 2 forcontrolling the energy of the signals supplied to the detector-amplier83 for operating the relay t9. The gain control apparatus 05 isconnected to a detector-amplifier 90 which in turn is connected to arelay lll. The relay El ope-rates an armature il@ for blocking thetransmission line when no signals are impressed thereupon. The gaincontrol apparatus 05 shown in Fig. 3 ot the drawings serves to insurethat no block on the transmission line will take place when signals areimpressed thereon so that no clipping of speech syllables can takeplace.

Modications in the circuits and in the arrangement and location ofparts-may be made within the spirit and scope of the invention and suchmodifications are intended to be covered by the appended claims.

What is claimed is:

l. In combination a signal line, means controlled by the signals on theline to hold the signals at substantially constant volume, control meansgoverned by signals on the line for clearing and blocking said line, andmeans controlled according to the energy levels of the signals on theline for selectively governing the energy level of the signals operatingsaid control means to minimize clipping.

2. In combination a signal line, a gain varying device on said line,means controlled by the signals on the line for governing said device tomain- 8. In combination a signal line, a gain varying device on saidline, means controlled by the signals on the line for governing saiddevice to maintain `the volume of the signals on the line beyond saiddevice substantially constant, normally operated blocking means on saidline beyond said device, control means operated by signals on said linefor removing the blocking means from the line, and means governed by twoenergy levels of the signals for maintaining the energy of the signalcurrents operating said control means at levels to minimize clipping.

Ll. In combination a signal line, a gain varying device on said line,means for controlling said device according to two different energylevels on said line to reduce the volume spread beyond the device andhold the signals at constant volume, means comprising an auxiliarycircuit controlled by the signals on the line before said devices andconnected to said line beyond the device therein for blocking the linewhen no signals are being transmitted and for removing the blockingmeans when signals are being transmitted, and means on said auxiliarycircuit controlled by the line signals for reducing the necessarysensitivity spread of the control currents on the auxiliary circuit tominimize clipping.

5. In combination a signal line, a gain varying device on said line,means controlled by the signals on. the line for governing said deviceto maintain constant volume on the line beyond said device, controlmeans governed by the signals on said line before said dev-ice forblocking the line beyond said device when no signals are on the line and`for insuring the free transmission. of signals when signals areimpressed on the line, and means for governing the signal currentsemployed to operate the control means to insure a small sensitivityspread so as to prevent clipping.

6. In combination a signal line, a gain varying device on said line,means for controlling said device according to two different energylevels on said line to maintain constant volume on the line beyond saiddevice, control means for blocking said line when nosignals are on theline and for effecting free transmission of signals when signals areimpressed on the line, and means governed according to two energy levelsof the signals on the line or adjusting the strength of the signalcurrents governing the control means to prevent clipping when signalsare on the line.

7. In combination a signal line, a gain varying device on said line,means controlled by the signals on the line for controlling said `deviceto maintain constant volume on the line, control means for blocking theline when free from signals and for removing the blocking on the linewhen signals are on the line, means controlled by two energy levels ofthe signals on the line for holding the energy level of the signalsoperating said control means between two levels to prevent clipping.

8. In combination a signal line, a gain varying device on said line,means controlled according to two energy levels of the signals on theline beyond said device for governing the device to maintain constantvolume on the line beyond said device, control means operated by linesignals for blocking said line beyond said device when the line is freefrom signals and for removing the line blocking when signals are on theline, and means controlled by the signals on the line before said devicefor holding the signal currents operating said control means betweenlevels to minimize clipping of the signals.

9. In combination a signal line, a gain varying device on said line,means controlled according to two energy levels of the signals on theline beyond said device for governing the device to maintain constantvolume beyond said device, control means operated by line signals forblocking said line beyond said device when the line is free from signalsand for removing the line blocking when signals are on the line, andmeans controlled by two energy levels of the signals on the line beforesaid device for governing the energy level of the signal currentsoperating said control means to minimize clipping of the signals on theline.

10. In combination a signal line, two gain adjusting devices on saidline, means controlled by the signals on the line for controlling therst device to maintain roughly constant volume on the line beyond thefirst device, means controlled by the signals on the line forcontrolling the second device to maintain constant volume on the linebeyond the second device, blocking means on the line between saiddevices for blocking the line when no signals are on the line, controlmeans operated by signals on the line between said devices for removingthe line blocking when signals are on the line, and means controlled bythe signals on the line between said devices for holding the signalcurrents operating said control lmeans between levels to minimizeclipping of the signals.

11. In combination a signal line, two gain adjusting devices on saidline, means controlled by the signals on the line for controlling therst device to maintain roughly constant volume on the line beyond therst device, means controlled by the signals on the line for controllingthe second device to maintain constant volume on the line beyond thesecond device, blocking means on the line between said devices forblocking the line when no signals are on the line, control meansoperated by signals on the line between said devices for removing theline blocking when signals are on the line, and means controlled by twoenergy levels of the signals on the line between said devices forgoverning the energy level of the signal currents operating said controlmeans to minimize clipping of the signals on the line.

12. In combination a signal line, control means governed by signals onthe line for blocking said line when no signals are on the line and forremoving the blocking when signals are on the line, and means governedby the energy levels of the signals on the line for selectivelycontrolling the energy level of the signals operating said control meansto minimize signal clipping 13. In combination a signal line, controlmeans governed by signals on the line for clearing and blocking theline, and means governed by the energy levels of the signals on the linefor selectively controlling the energy level of the signals operatingsaid control means to minimize signal clipping.

BJ ORN G. BJORNSON.

